Alkali salt electrolysis by mercury process

ABSTRACT

In an alkali salt electrolysis operation by the mercury process wherein mercury recovered in a denuding tower is recirculated with wash water through a mercury return passageway to be returned to an electrolytic cell, a portion of the mercury is caused to constantly overflow together with washing water over a weir provided in the mercury return passageway and then to undergo auxiliary circulation to a mercury reservoir interposed between the denuding tower and a mercury pump. An alkali chloride electrolytic apparatus, in which a reservoir tank is provided between a denuding tower and a mercury pump and an auxiliary circulation passageway communicates between a mercury return passageway and an entrance of the reservoir tank.

United States Patent Shibata et a1.

[15] 3,663,385 [451 May 16, 1972 [54] ALKALI SALT ELECTROLYSIS BY MERCURY PROCESS [72] Inventors: Hiroshi Shibata; Teruo lmai; Shigeji Kumaki, all of Iwaki-shi, Japan Kureha Kagaku Kogyo Kabushiki Kaisha, Tokyo-to, Japan [22] Filed: Aug. 25, 1969 [21] Appl.No.: 852,582

[73] Assignee:

[30] Foreign Application Priority Data Aug. 30, 1968 Japan ..43/61725 [52] US. Cl ..204/99 [51] Int. Cl. .C0ld 3/00 [58] Field of Search... ..204/99 [56] References Cited UNITED STATES PATENTS 2,748,072 5/1956 Paoloni et a1. ..204/99 2,815,318 12/1957 Shaw et a1... ..204/99 2,824,054 2/1958 Crabbe ..204/99 3,091,579 5/1963 Basilevsky ..204/99 3,398,075 8/1968 Shibata 204/99 3,420,757 1/1969 Friemel et al ..204/99 Primary Examiner-Winston A. Douglas Assistant Examiner-H. A. Feeley Attorney-Wenderoth, Lind & Ponack ABSTRACT In an alkali salt electrolysis operation by the mercury process wherein mercury recovered in a denuding tower is recirculated with wash water through a mercury return passageway to be returned to an electrolytic cell, a portion of the mercury is caused to constantly overflow together with washing water over a weir provided in the mercury return passageway and then to undergo auxiliary circulation to a mercury reservoir interposed between the denuding tower and a mercury pump.

An alkali chloride electrolytic apparatus, in which a reservoir tank is provided between a denuding tower and a mercury pump and an auxiliary circulation passageway communicates between a mercury return passageway and an entrance of the reservoir tank.

1 Claims, 3 Drawing Figures Patented May 16, 1972 lllHlllH FIG. 3

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MJQMAMm BACKGROUND OF THE INVENTION This invention relates to an improvement in alkali chloride electrolysis by mercury process, and more particularly to an improved alkali chloride electrolysis, wherein wash water and a small amount of mercury which have been separated prior to its return to the electrolytic cell are caused to by pass said cell and be recirculated to a mercury reservoir provided between a denuding tower and a mercury pump.

In the applicants US. application Ser. No. 715046, now US. Pat. No. 3,607,704 issued of Sept. 21, 1971, it has been proposed that, before the mercury which has been recovered in the denuding tower is sent by a pump into the electrolytic cell, most of it is separated by an underflow baffle plate having at the lower part thereof a mercury passage for separating mercury alone and which baffle plate is provided in the main circulation passageway for mercury at a position just before the entrance of the electrolytic cell, and contaminants such as mercury emulsion, etc. Floating in the path on the upstream side of the baffle plate are separated along with the washing water and a small amount of mercury and are caused to be recirculated in an auxiliary circulation passageway for alkali metal amalgam flowed out of the electrolytic cell and separated from the electrolyte and amalgam butter, and then sent into the denuding tower, thereby accomplishing perfect separation of mercury from contaminants.

According to the invention as disclosed in said prior application, it has become possible to save additional labor for scooping off the mercury emulsion as well as to prevent the mercury emulsion which accumulates on the up-stream side of the baffle plate from being drawn and swept along with the mercury passing under the baffle plate and entering into the electrolysis bath vessel to become intermixed with the electrolyte. However, there still remain some problem with the invention. That is, although the water for washing is ordinarily supplied direct to the inlet of the mercury pump so as to perform effective washing of the mercury due to the stirring action of the mercury pump, the washing water and the mercury, due to difference in their specific gravity, are easily separated into two layers with the result that the quantity of the washing water passing through the pump varies greatly with lapse of time, and this separated water constantly varies the supplying quantity thereof into the denuding tower when it is separated at the inlet of the electrolytic cell and directly enters the denuding tower. This variation in the washing water affects the concentration tower. This variation in the washing water affects the concentration of caustic soda produced at the denuding tower, and moreover causes considerable trouble in cases where, for example, the water supply to the denuding tower is to be automatically adjusted in accordance with the concentration of caustic soda at the exit side of the denuding tower.

SUMMARY OF THE INVENTION In order to solve this problem, the present invention proposes an improved and more effective alkali chloride electrolysis by mercury process, in which the washing water and a small quantity of mercury separated before it can enter the electrolytic cell are caused to circulate into a cushion box provided between the denuding tower and the mercury pump. The cushion box is a reservoir for temporarily storing mercury to be sent from the denuding tower to a mercury pump, as well as for storing some of the washing water.

BRIEF DESCRIPTION OF DRAWING The inventive feature of the present application will become more apparent from the following description of the invention when read in conjunction with the accompanying drawing, in which:

FIG. 1 is a plan view showing one embodiment of the alkali chloride electrolytic apparatus according to the present invention;

FIG. 2 is a plan view showing another embodiment of the present invention; and

FIG. 3 is a fragmentary, relatively enlarged perspective view, with parts cut away and parts in section, showing the part encircled by a dotted line and designated by Roman numeral III in FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, the mercury which has flown through an electrolytic cell 1 and turned to sodium amalgam passes through a gas baffle 2 and a mercury baffle 3, and then is separated into the electrolyte and amalgam butter, thereafter it enters into a denuding tower, where it is denuded to produce caustic soda, while recovering mercury. The mercury recovered at the denuding tower 4 is further sent to a pump 7 through a reservoir 5 where it gets together with washing water fed thereinto from the water supply passageway 6. In the pump 7, the mercury and the washing water are stirred and mixed, whereby the mercury is washed and cooled. The mercury and wash water discharged from the pump 7 enter into a mercury return passageway 8, through which they are sent to a mercury separation passageway 9. This passageway 9 is of such a construction as shown in FIG. 3, for example also shown in co-pending application Ser. No. 715,046. That is, as is clear from the drawing, the passageway 9 possesses one of the side walls thereof in common with the back side wall of the electrolytic cell 1, and are provided along the lower part of the common side wall a plurality of openings 11 for the separated mercury to flow inside the electrolytic cell. Across the downstream end of the passageway 9, there is provided a weir 12, by which the mercury accumulates up to the height of this weir l2 and the mercury separated from the wash water at this place enters into the electrolytic cell 1 through the inlet openings 11. At this time, a portion of the mercury accumulated in this passageway together with mercury emulsion and wash water floating thereabove overflow over the weir 12 to fall into an auxiliary circulation passageway 13 connected to the reservoir 5, and is re-circulated into the reservoir 5. In this case, since the wash water is to circulate in the progressive order of the reservoir, the mercury pump 7, the mercury return passageway 8, the mercury separation passageway 9, and the auxiliary circulation passageway 13, the wash water is supposed to increase gradually its alkalinity. In order to avoid this tendency, a water supply passageway 14 connecting the reservoir 5 and the denuding tower 4 via a water pump 15 is provided so as to send a portion of the first mentioned washing water, which enter at 6, to the denuding tower.

According to the present invention, since no manual scooping off of mercury emulsion is required, laborious work can be considerably reduced and, moreover, there is no loss of mercury to be recovered at the refining stage of the mercury emulsion thus scooped off. Furthermore, as a small quantity of mercury constantly overflows together with the wash water, the surface of the mercury in front of the weir 12 is always kept free of the wash water and the mercury emulsion floating on the top part of the mercury and accompanying therewith at the time when it flows in the passageway 9 and passes into the electrolysis bath vessel through the openings 1 1.

In the present invention, no mercury emulsion is scooped off from its circulation passageway, because, unlike amalgam butter, it can be relatively easily separated into mercury and water, and there is no apprehension that a large quantity of the mercury emulsion accumulates while it is being recirculated in the auxiliary circulation passageway 13 alone. Also, as the graphite particles, etc. in the mercury emulsion are relatively light and therefore is floatable in water, it can be returned to the denuding tower 4 together with a portion of the washing water to be sent thereto from the reservoir 5, hence no accumulation of the graphite particles.

It is understood heretofore that most of the impurities such as graphite particles, etc. produced within the denuding tower have been taken out together with caustic soda produced in the denuding tower (the outlet pipe not being shown in the drawing), and both impurities and caustic soda are separated by means of filtration and other expedient means so that the impurities which have been sent into the denuding tower by the water pump and are circulating in the denuding tower can be similarly removed. in this consequence, no foreign substance, such as graphite particles, etc., accumulate in the circulation passageway which connects in series the surge tank, the mercury pump, and the mercury separation passageway.

The denuding tower or denuder, as referred to herein, is intended to be only a vertical type denuder, and is not any arbitrary type.

In the aforementioned US. application Ser. No. 715,046 now U.S. Pat. No. 3,607,704, the mercury emulsion and a portion of the mercury, etc. which have been subjected to the auxiliary circulation enter the denuding tower through the exit of the electrolysis bath vessel. In the present invention, however, the recirculated mercury does not enter the denuding tower 4, hence the quantity of mercury to be recirculated can be reduced and at the same time the operating efficiency of the denuding tower is improved. Moreover, when the wash water is to be returned to the denuding tower, although the difference in head between the entrance of the electrolysis bath vessel and the entrance of the denuding tower is small, if the mercury emulsion is sent back to the reservoir 5 disposed at a lower position than the denuding tower 4, a large head can be obtained, so that the flow of the wash water over the weir 12 becomes great and the separation of the mercury from the mercury emulsion at this portion is effectively improved.

The present invention is not limited to the mercury circulation passageway as shown in FIG. 1, but also is applicable respectively to the horizontal electrolytic cell of a single passageway type, a mercury circulation passageway of an electrolysis bath vessel like that shown in FIG. 2 where the mercury emulsion flows down across the width of along electrolytic cell, and/or an electrolytic cell of a bent-type.

In the embodiment shown in FIG. 2, mercury which has flowed down across the electrolytic cell 21 in the arrowed direction and turned into mercury amalgam passes through the passage 22 and is discharged outside of the vessel through a mercury baffle 23 where it is separated into the electrolyte and amalgam butter, after which the electrolyte and the amalgam butter thus separated enter into a denuding tower 25. The amalgam butter which is stagnant in front of the mercury baffle 23 is removed by an appropriate expedient.

The mercury which is discharged from the denuding tower 25 enters into a reservoir 27 where it is mixed with water fed from a water supply pipe 27a and the wash water circulated through an auxiliary circulation passageway 31, and then is sent into a circulation passageway 28 by means of a mercury pump 26. Another mercury baffle 29 is provided at a side wall of the circulation passageway 28, at the extreme end of which there is provided a weir 30 communicating to the auxiliary circulation passageway 31. Most of mercury flowing in this circulation passageway 28 passes through the mercury baffle 29 to enter into the other circulation passageway 32, from which it is recirculated into the electrolysis bath vessel 21. In this case, as a portion of the mercury constantly overflows over the weir 30 together with the mercury emulsion and the wash water, the surface part of the mercury even at the portion along the mercury baflle 29 has a flowing direction toward the weir 30, such that the mercury at the surface part and the mercury emulsion and washing water flowing thereover do not intermix with the flow of mercury passing under the baffle 29, thereby accomplishing perfect mercury separation. Also, a portion of the water in the reservoir tank 27 is sent to the denuding tower 25 through a pipeline 33 by means of a pump 34, and is used as a denuding water,

In this connection, water to be fed from outside for use as washing water is also used ultimately as denuding water, so that it is desirable to use water of high purity such as deionized water, etc., and the place where it is to be supplied is not only not limited direct to the reservoir tank, but it may be fed to any place in the circulation circuit for use as the washing water.

We claim:

1. In an alkali chloride electrolysis process using a mercury electrode in conjunction with other apparatus, wherein mercury is recovered at a denuding tower, circulated by a mercury pump together with a mixture of separately introduced washing water and mercury butter, and then is separated substantially into the mercury and wash water by means of a weir provided at the bottom of a side wall of a mercury separating passageway in a mercury circulation conduit, to divert the separated mercury alone into one side of an electrolysis vessel which is connected at its opposite side with the denuding tower, the improvement in the process which comprises:

a. temporarily storing mercury leaving the denuding tower enroute to said mercury pump in a reservoir which is interposed in the circulation conduit of the apparatus between the denuding tower and the mercury pump, which reservoir is to be used for collecting and storing the mercury, and also for storing some of the washing water which is initially introduced from a fresh supply thereinto;

b. positioning an overflow weir transversely in the bottom part of and at the downstream end of a perforated side wall of a mercury separating passageway so that a majority of the mercury is separated and directed into said electrolysis vessel with the relatively lesser quantity of the mercury overflowing the weir to be recirculated with said wash water;

c. circulating the relatively smaller quantity of the mercury together with wash water so as to bypass the electrolysis vessel and directing it into said reservoir rather than to said denuding tower by circulating it through a bypass conduit interconnecting said mercury separating passageway and said reservoir positioned beyond said denuding tower; and

d. directing a portion of the washing water, which was initially introduced into the reservoir, to said denuding tower to prevent the washing water in the circulation conduit from acquiring too strong alkalinity.

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